The article presents an analysis of the evolution of explanations of the nature of social inequality in sociology. The concepts of E. Durkheim, T. Parsons, K. Marx, M. Weber,P. Bourdieu and the modern Russian economist-sociologist V. Radaev are analyzed. it is concluded that modern sociology has come to the realization that social inequalities are formed on the basis of the unequal possession by members of society of various forms of capital. Forms of capital, which have the ability to convert into each other, are the basis of not only social inequality, but also social mobility.